Energy conservation through mandated energy efficiencies and straightforward cutbacks in usage by the public and private sectors can clearly generate important fuel savings. However, really significant national gains can be realized only through timely development and implementation of a range of advanced technology concepts that promise very high efficiencies. Turbine engines are widely used for many different applications but, in the past, this use has been almost entirely limited to engines of the higher power range, e.g. greater than 1000 kW.
In recent years the most successful approach taken to extend the application of the turbine to very small power levels has been to utilize a high-molecular-weight vapor as the working fluid. This approach has been effective down to power levels of about 10 kW (bottoming cycles in truck engines). However, the turbine sizes become very small and the rotational velocities very high. Therefore, to operate below this level, additional approaches have been taken, such as the application of less-efficient, partially-admitted turbines and/or extremely low working pressures. These approaches have not been fully satisfactory and generally have been limited to special applications.
Two problem areas have been dominant in the development of high-efficiency, very-low-power turbines. One has been the problem of excessive heat loss in the high temperature section, resulting in reduced operating efficiency. The other has been the adverse effects of small turbine diameters, e.g., low efficiency and extremely high rpm.
An ejector used to pump a fluid is usually thought of as inefficient since that has been the case in most previous applications. However, U.S. Air Force studies (Lawson, M. O., "Electrofluid Dynamic Generators and Cycle Performance for Optimally Matched Ejectors," AFAPL-TR-76-111, Wright-Patterson Air Force Base, Ohio, July 1976; Huberman, M., et al., "Study on Electrofluid Dynamic Power Generation," ARL-TR-75-0200, June 1975) indicate flow transfer efficiencies in excess of 90 percent for high-volume-flow-ratio ejectors that meet two major operating criteria. First, the two flows have nearly equal speed at mixing, and second, the Mach number of the larger volume flow, which is being "pumped," should be low subsonic. For a high-pressure-ratio cycle (e.g., a Rankine cycle), both of these conditions can be met by the selection and matching of the two working fluids: one is the thermodynamic fluid which should be of high molecular weight and the other is the turbine cycle fluid which should be of extremely low molecular weight, e.g., hydrogen or helium.